Evaluation of the Prompt inoculation system for preparation of standardized bacterial inocula

Abstract

Standardized inocula prepared with the 3M Prompt inoculation system were evaluated by (i) performing colony counts on the inocula and (ii) employing these inocula in disk diffusion or broth microdilution susceptibility tests. The inoculation wand delivered 1.85 X 10(8.0) +/- 0.6 log10 CFU to the various diluents tested. Inocula prepared in 1 ml of saline and used to inoculate disk diffusion tests resulted in 96.5% agreement between Prompt and standard tests when Prompt inocula were used within 15 min of preparation and 95.6% agreement between paired tests when Prompt inocula were allowed to sit at room temperature for 2 h before use. With 15-min Prompt inocula, only one (0.04%) major discrepancy between paired tests was observed. The 2-h Prompt inocula gave three (0.13%) major or very major discrepancies. The 15-min Prompt inocula prepared in 30 ml of diluent resulted in 98.2% of Prompt minimal inhibitory concentrations within +/- 1 log2 dilution step of the standard test result, whereas the 2-h inocula resulted in 97.4% agreement between paired minimal inhibitory concentrations at this level. Thus, the Prompt inocula were found to give results equivalent to those obtained from inocula prepared by conventional procedures.